How the Oxy-Fuel Cutting Process Works
Oxy-Fuel Cuts What?
The oxy-fuel cutting process is capable of cutting steel whose composition is less than .3% carbon. Mild steel is comprised of 98% iron, 1% manganese, carbon up to .3% and various other elements in small amounts. Oxy-fuel cutting cannot cut non-ferrous metals such as aluminum, stainless, brass or copper. Elements such as chromium, nickel, molybdenum inhibit the ability to cut steel with the oxy-fuel process. Manganese, silicon, phosphorus, and sulfur in normal amounts have little effect on the cutting process.
Heat and Oxidation
It’s a simple matter of heat and added oxygen resulting in rapid oxidation. Oxy-fuel cutting torches use a copper tip with a series of holes arranged in a circle to deliver a mixture of a fuel gas and oxygen to create a pre-heat flame. Preheat flame temperatures will range from 4,440° to 6,000° F depending upon the fuel gas used and the adjusted ratio of oxygen to fuel gas.
Mechanized oxy-fuel machine torches, which are normally either 10” or 18” in length, are typically rated for cutting up to 12” to 15”. Some specialty machine torches are rated for cutting up to 30” and more. Cutting thick plate does not require high oxygen pressures. However, cutting thick plate does require a much higher volume of oxygen. Also, the hoses and gas manifold must be designed to deliver the required flows. For example, a cutting machine designed for multiple torch thick plate cutting will typically be configured with 1” I.D. oxygen supply hoses.
Natural gas (if available with enough pressure) will deliver excellent performance and the lowest cost per foot of cut. Alternatively, propane will produce excellent results as well. If cutting low alloy steel, propylene or propylene mixes should be considered.
Acetylene outperforms the other gases on thin plate (less than ¼”). However, this segment of the market was taken over in the 1980s by plasma and then by laser. Additionally, acetylene is far and away the most expensive fuel gas per cubic foot. It also requires numerous cylinders to be manifolded together to provide the required flow for multi torch operations.
The AWS Handbook states:
- The oxygen used for cutting must have a purity of 99.5% or higher
- Lower purity reduces the efficiency of the cutting operation
- A 0.1% decrease in oxygen purity will reduce cutting speed by 10%
- Low purity oxygen also increases the amount and tenacity of the slag to adhere
- Oxygen purity below 95% results essentially in an unacceptable melt-and-wash action
Single and Two-Piece Tips
Single piece cutting tips are normally designed for cutting with acetylene. Single piece tips are copper with a drilled and swaged hole for the center cutting oxygen flow and four to six drilled holes for the preheat gas flow.
Two-piece tips are designed with an outer copper shell and an inner insert with multiple machined splines to deliver the preheat gases. Two-piece cutting tips are normally designed for cutting with alternative fuels such as natural gas and propane.
The Right Tip
Each torch tip manufacturer provides charts specifying the proper size tip based on material thickness. Below is a chart from Harris for their style 98-6 torch when using either propane or natural gas as the fuel. Premium results require that the cut chart be carefully followed
Straight and Divergent Bore
Straight bore tips typically use between 40 – 60 PSI of cutting oxygen. Machine cutting tips are typically divergent bore tips with a tapered or flared outlet hole. Divergent bore tips use 70 – 100 PSI of cutting oxygen and offer 25% increase in cutting speed over straight-bore tips.
Once the preheat gases are ignited, the preheat flame is adjusted to a neutral flame (equal amount of fuel and oxygen), an oxidizing flame (excess amount of oxygen) or a carburizing flame (excess amount of fuel). Oxy-fuel cutting is normally done with a neutral flame.
“Walking Up” the Flame
The preheat flame is adjusted to produce a softer or more aggressive flame by increasing the amount of both fuel and oxygen. Some refer to this technique as “walking up” the flame. Aggressive preheat flames more quickly bring the metal to its kindling temperature.
Basic systems offer manual up/down adjustment of torch lifter by the operator. Advanced systems automatically control the torch lifter via an electronic circuit measuring capacitance or inductance between torch and metal being cut. The torch tip is positioned above the plate at the proper height for the fuel gas in use.
Acetylene preheat flames are set just above the plate surface for maximum heat input. Natural gas and propane preheat flames are set approximately ¾” above the plate for maximum heat input.
CNC PLASMA CUTTING MACHINE
If the high preheat flame is very soft, time to reach the kindling temperature may take up to 2 minutes. If the high preheat flame is sufficiently aggressive, preheat time on ½” to 6” material can be as little as 10 seconds.
The pre-heat flames should make a “star” pattern on the plate during pre-heating the plate before the cutting oxygen is turned on – as shown below.
The high purity oxygen stream causes the steel to rapidly oxidize and this reaction is highly exothermic. An exothermic reaction is a chemical reaction that releases energy by light or heat. The oxygen stream causes the steel to actually catch fire just like paper or wood. Oxygen flow to cut a given thickness of steel is the same regardless which fuel gas is used. Too little oxygen causes a slow and ragged cut. Too much oxygen causes a wide concave cut.
The operator must select the proper fuel gas for the task and the correct size cutting tip. The source of gases and hoses (both fuel gas and oxygen) must be capable of delivering the required flow at the prescribed pressure. Oxygen and fuel gas pressure must be set correctly. The preheat flames (low and high) must adjusted to the proper ratio of oxygen to the fuel gas. The cutting speed must be set according to the cut chart. The pierce height, preheat time and pierce time must be set. The correct torch tip to work distance must be maintained during cutting.
CNC Process Control
Basic systems require the machine operator to manually adjust pressures, pierce rate control and process event timing for each job. However, increasingly today’s CNC controls include embedded cut charts to prompt the machine operator how to perform the cutting. Once the machine operator selects the torch manufacturer, material thickness and fuel gas to be used, the cut chart typically displays suggested cutting speed, gas pressures, the timing of process events (preheat time, pierce time) and the correct size torch tip.
The gas management system should provide the ability to individually control pressures for low preheat fuel, low preheat oxygen, high preheat fuel, high preheat oxygen and cutting oxygen. It should also control the rate of rise of the cutting oxygen during piercing to minimize excessive spatter during the pierce. Pierce rate control is especially important when piercing steel thicker than 2”.
Gas Pressures Set by the CNC
Hypertherm CNCs provide analog outputs that can control proportional gas regulators as shown in the diagram below. Using this strategy allows you to reduce the number of regulators needed for the gas delivery system from the normal six to three as shown below. Analog outputs are available for three channels: fuel gas, preheat oxygen, and pierce/cut oxygen.
Adjusting Pressures & Timers
When adjusting the gas pressures or timers, use the Apply soft key to send the pressures to the cutting system. As you are fine-tuning the system for your requirements, you can select Apply, change the pressures on the screen, and select Apply again without exiting the screen.
Auto Height Control and Internal Ignition
The IHT FIT + Three torch and control system offers ability to automatically maintain torch to plate height as well as ignition of the gases upon command from the CNC. The system controls preheat height, pierce height, cutting height and retract height after the cut.
Oxy-Fuel Cutting Solutions
KANO™ HDCNC Plasma Cutting
KANO™ HD is a true unitized CNC cutting machine available in six machine sizes configured with a Hypertherm plasma system and your choice of three cutting table styles.
TracKlean™Self-Cleaning Table with AirSweep
The TracKlean table is comprised of two unique traveling cars, each with a slag bin and a detachable plow for fast part retrieval and easy cleaning.
RoboVent FusionFume Extraction Collector Series
As the recommended fume extractor for our plasma cutting machines, the Fusion Collectors from RoboVent offer perfect air quality solution for cutting-edge processes.
Zoned Down Air DraftCutting Table
The Zoned Down Air Draft table with mechanically activated zone damper doors offers the cut quality benefits of cutting dry and excellent fume extraction.
More About Metal
Industries that use our cutting solutions.
Optimizing Plasma CutTING
A guide to optimizing your plasma cutting.
Plasma Cutting 101
Understand the plasma cutting process.
History of Plasma
An extensive timeline of plasma cutting.
Understand the oxy-fuel cutting process.
History Of Oxy-Fuel
The origins of oxy-fuel.
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